1. Field of the Invention
The present invention relates to a novel process for preparing episulfide resins via reactive melt extrusion of converted epoxy resins.
2. Description of Background and Related Art
It is known to produce episulfide resins via a solution process by carrying out the reaction of epoxy resins such as for example novolacs and sulfur donating compounds such as for example, thiourea and thiourea-derivatives, potassium thiocyanate, thiocyanate anions with various counter cations including sodium, potassium, Group I elements, Group II elements, and phosphine sulfides for example as described in U.S. Pat. No. 6,551,711.
The episulfide resins are common in industrial applications where a low temperature cure and/or shorter gel times of the resins are needed. Temperature of cure and gel time typically depends on the end use application of the resin. For example marine coatings applications can be cured as low as (about 5° C.) and powder coatings applications may be cured as low as at about 120° C.
However, known processes for making episulfide resins use primarily a solvent-based reaction media as described in JP2001-41458, JP2000007759, JP 11140161, and JP 11140188. A reaction process using a solvent-based reaction media generates volatile organic compound (VOC) waste and takes a substantial amount of time (for example, days at ambient reaction temperatures, hours at temperatures up to 70° C.) for a significant conversion to occur (e.g. greater than 60% conversion). In addition, episulfide resins are known to be less stable (as the episulfide has been described as a complex between sulfur atom and alkenes; see for example, Dittmer, D C., “Thiiranes and Thiirenes” in “Comprehensive Heterocyclic Chemistry”, Vol. 7, W. Lwowski, Ed., Pergamon, Oxford, 1984, Ch. 5.06, pp. 131-184) than analogous epoxy resins and hence the episulfide resins have a relative short shelf-life (e.g. less than 1 month).
It is desired to provide a process that eliminates the need for solvent which greatly reduces VOC waste; and a process that addresses the storage stability issue commonly found in handling episulfide resins.